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Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <20210112214337.GA10434@willie-the-truck>
Date:   Tue, 12 Jan 2021 14:29:51 -0800
Cc:     Yu Zhao <yuzhao@google.com>,
        Laurent Dufour <ldufour@linux.vnet.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Vinayak Menon <vinmenon@codeaurora.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Xu <peterx@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
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        Pavel Emelyanov <xemul@openvz.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        stable <stable@vger.kernel.org>,
        Minchan Kim <minchan@kernel.org>, surenb@google.com
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To:     Will Deacon <will@kernel.org>
Precedence: bulk

> On Jan 12, 2021, at 1:43 PM, Will Deacon <will@kernel.org> wrote:
> 
> On Tue, Jan 12, 2021 at 12:38:34PM -0800, Nadav Amit wrote:
>>> On Jan 12, 2021, at 11:56 AM, Yu Zhao <yuzhao@google.com> wrote:
>>> On Tue, Jan 12, 2021 at 11:15:43AM -0800, Nadav Amit wrote:
>>>> I will send an RFC soon for per-table deferred TLB flushes tracking.
>>>> The basic idea is to save a generation in the page-struct that tracks
>>>> when deferred PTE change took place, and track whenever a TLB flush
>>>> completed. In addition, other users - such as mprotect - would use
>>>> the tlb_gather interface.
>>>> 
>>>> Unfortunately, due to limited space in page-struct this would only
>>>> be possible for 64-bit (and my implementation is only for x86-64).
>>> 
>>> I don't want to discourage you but I don't think this would end up
>>> well. PPC doesn't necessarily follow one-page-struct-per-table rule,
>>> and I've run into problems with this before while trying to do
>>> something similar.
>> 
>> Discourage, discourage. Better now than later.
>> 
>> It will be relatively easy to extend the scheme to be per-VMA instead of
>> per-table for architectures that prefer it this way. It does require
>> TLB-generation tracking though, which Andy only implemented for x86, so I
>> will focus on x86-64 right now.
> 
> Can you remind me of what we're missing on arm64 in this area, please? I'm
> happy to help get this up and running once you have something I can build
> on.

Let me first finish making something that we can use as a basis for a
discussion. I do not waste your time before I have something ready.